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Medieval artistic representation of a spherical Earth - with compartments representing earth, air, and water (c.1400).

The concept of a sphericalEarth dates back to ancient Greek philosophy from around the 6th century BCE,[1] but remained a matter of philosophical speculation until the 3rd century BC when Hellenistic astronomy established the spherical shape of the earth as a physical given. The Greek paradigm was gradually adopted, either directly or via later European science, by all major civilizations and world regions, becoming the dominant concept of modern times.[2][3][4][5] The final, practical evidence of the spherical shape of the earth provided its circumnavigation by the Portuguese explorer Ferdinand Magellan (1519−1521).

As determined by modern instruments, a sphere approximates the Earth's shape to within one part in 300. An oblate ellipsoid shape with a flattening of 1/300 matches even more precisely. Recent measurements from satellites suggest that the Earth is, in fact, slightly pear-shaped.[8]

Contents

History

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Early Greek and Hellenistic development

Though the earliest evidence of a spherical Earth comes from ancient Greek sources, there is no account of how the sphericity of the Earth was discovered. A plausible explanation is that it was "the experience of travellers that suggested such an explanation for the variation in the observable altitude and the change in the area of circumpolar stars, a change that was quite drastic between Greek settlements" around the eastern Mediterranean Sea, particularly those between the Nile Delta and the Crimea.[9]

Pythagoras

Early Greek philosophers alluded to a spherical Earth, though with some ambiguity.[10] This idea influenced Pythagoras (b. 570 BCE), who saw harmony in the universe and sought to explain it. He reasoned that Earth and the other planets must be spheres, since the most harmonious geometric solid form is a sphere.[1] After the fifth century BCE, no Greek writer of repute thought the world was anything but round.[10]

Herodotus

In The Histories, written 431 BCE - 425 BCE, Herodotus dismisses a report of the sun observed shining from the north. This arises when discussing the circumnavigation of Africa undertaken c. 615-595 BCE. (The Histories, 4.43) His dismissive comment attests to a widespread ignorance of the ecliptic's inverted declination in a southern hemisphere.

Plato

Plato (427 BCE - 347 BCE) travelled to southern Italy to study Pythagorean mathematics. When he returned to Athens and established his school, Plato also taught his students that Earth was a sphere. If man could soar high above the clouds, Earth would resemble "one of those balls which have leather coverings in twelve pieces, and is decked with various colours, of which the colours used by painters on earth are in a manner samples." (Phaedo, 110b)

Aristotle

When a ship is at the horizon its lower part is invisible due to Earth's curvature. This was one of the first arguments favoring a round-Earth model.

Aristotle (384 BCE - 322 BCE) was Plato's prize student and "the mind of the school." Aristotle observed "there are stars seen in Egypt and [...] Cyprus which are not seen in the northerly regions." Since this could only happen on a curved surface, he too believed Earth was a sphere "of no great size, for otherwise the effect of so slight a change of place would not be quickly apparent." (De caelo, 298a2-10)

Aristotle provided physical and observational arguments supporting the idea of a spherical Earth:

Every portion of the Earth tends toward the center until by compression and convergence they form a sphere. (De caelo, 297a9-21)

Travelers going south see southern constellations rise higher above the horizon; and

The shadow of Earth on the Moon during a lunar eclipse is round. (De caelo, 297b31-298a10)

The concepts of symmetry, equilibrium and cyclic repetition permeated Aristotle's work. In his Meteorology he divided the world into five climatic zones: two temperate areas separated by a torrid zone near the equator, and two cold inhospitable regions, "one near our upper or northern pole and the other near the ... southern pole," both impenetrable and girdled with ice (Meteorologica, 362a31-35). Although no humans could survive in the frigid zones, inhabitants in the southern temperate regions could exist.

Eratosthenes

Eratosthenes (276 BCE - 194 BCE) estimated Earth's circumference around 240 BCE. He had heard that in Syene the Sun was directly overhead at the summer solstice whereas in Alexandria it still cast a shadow. Using the differing angles the shadows made as the basis of his trigonometric calculations he estimated a circumference of around 250,000 stades. The length of a 'stade' is not precisely known, but Eratosthenes' figure only has an error of around five to ten percent.[11][12]

It has been suggested that seafarers probably provided the first observational evidence that the Earth was not flat, based on observations of the horizon. This argument was put forward by the geographer Strabo (c. 64 BC – AD 24), who suggested that the spherical shape of the Earth was probably known to seafarers around the Mediterranean Sea since at least the time of Homer.[13]

The first part of the Geographia is a discussion of the data and of the methods he used. As with the model of the solar system in the Almagest, Ptolemy put all this information into a grand scheme. He assigned coordinates to all the places and geographic features he knew, in a grid that spanned the globe. Latitude was measured from the equator, as it is today, but Ptolemy preferred to express it as the length of the longest day rather than degrees of arc (the length of the midsummer day increases from 12h to 24h as you go from the equator to the polar circle). He put the meridian of 0 longitude at the most western land he knew, the Canary Islands.

Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (oikoumenè) and of the Roman provinces. In the second part of the Geographia he provided the necessary topographic lists, and captions for the maps. His oikoumenè spanned 180 degrees of longitude from the Canary Islands in the Atlantic Ocean to China, and about 81 degrees of latitude from the Arctic to the East Indies and deep into Africa. Ptolemy was well aware that he knew about only a quarter of the globe.

Global diffusion

Spread to the west

Spherical earth with the four seasons. Illustration in 12th century book Liber Divinorum Operum by Hildegard of Bingen

From its Greek origins, the idea of a spherical earth, along with much of Greek astronomical thought, slowly spread across the globe and ultimately became the adopted view in all major astronomical traditions:[2][3][4][5] In the west, the idea came naturally to the Romans through the lengthy process of cross-fertilization with Greek civilization. Many Roman authors such as Cicero and Pliny refer in their works to the rotundity of the earth as a matter of course.[14]

In the 17th century, the idea of a spherical earth, now considerably advanced by Western Astronomy, ultimately spread to Ming China, when Jesuit missionaries, who held high positions as astronomers at the imperial court, successfully challenged the Chinese belief that the earth was flat and square.[18][19][20]

Other ancient views

Rigveda

While Indian cosmology long maintained the belief in a disk-shaped earth, K. V. Sarma mentions in passing that the Rigveda contains speculations on the idea of a spherical self-supporting earth, but does not give more details nor references.[21] It is impossible to date when any particular piece of the Rigveda was composed. While the core of the text probably dates to around 1400 BCE, many revisions and redactions are thought to have been incorporated around 600 BCE.[22] Nothing was written down until the 3rd century BCE at earliest, and possibly much later. See Dating and historical context of the Rigveda.

Aryabhata

The works of the classical Indian astronomer and mathematician, Aryabhata (476-550 AD), deal with the sphericity of the Earth and the motion of the planets. The final two parts of his Sanskrit magnum opus, the Aryabhatiya, which were named the Kalakriya ("reckoning of time") and the Gola ("sphere"), state that the Earth is spherical and that its circumference is 4,967 yojanas, which in modern units is 39,968 km, which is only 62 km less than the current value of 40,030 km.[23][24] He also stated that the apparent rotation of the celestial objects was due to the actual rotation of the Earth, calculating the length of the sidereal day to be 23 hours, 56 minutes and 4.1 seconds, which is also surprisingly accurate. It is likely that Aryabhata's results influenced European astronomy, because the 8th century Arabic version of the Aryabhatiya was translated into Latin in the 13th century.

Anania Shirakatsi

Anania Shirakatsi (Armenian: Անանիա Շիրակացի), also known as Ananias of Sirak, (610–685) was an Armenianscholar, mathematician, and geographer. His most famous works are Geography Guide (‘Ashharatsuyts’ in Armenian), and Cosmography (Tiezeragitutiun). He described the world as "being like an egg with a spherical yolk (the globe) surrounded by a layer of white (the atmosphere) and covered with a hard shell (the sky)." [25]

Around 830 CE, Caliph Al-Ma'mun commissioned a group of Muslim astronomers and Muslim geographers to measure the distance from Tadmur (Palmyra) to al-Raqqah, in modern Syria. They found the cities to be separated by one degree of latitude and the distance between them to be 66 2/3 miles and thus calculated the Earth's circumference to be 24,000 miles.[28]

Another estimate given by his astronomers was 56 2/3 Arabic miles per degree, which corresponds to 111.8 km per degree and a circumference of 40,248 km, very close to the currently modern values of 111.3 km per degree and 40,068 km circumference, respectively.[29]

Al-Farghānī

Al-Farghānī (Latinized as Alfraganus) was a Persian astronomer of the 9th century involved in measuring the diameter of the Earth, and commissioned by Al-Ma'mun. His estimate given above for a degree (56 2/3 Arabic miles) was much more accurate than the 60 2/3 Roman miles (89.7 km) given by Ptolemy. Christopher Columbus uncritically used Alfraganus's figure as if it were in Roman miles instead of in Arabic miles, in order to prove a smaller size of the Earth than that propounded by Ptolemy.[30]

Biruni

Abu Rayhan Biruni (973-1048) solved a complex geodesic equation in order to accurately compute the Earth's circumference, which was close to modern values of the Earth's circumference.[31][32] His estimate of 6,339.9 km for the Earth radius was only 16.8 km less than the modern value of 6,356.7 km. In contrast to his predecessors who measured the Earth's circumference by sighting the Sun simultaneously from two different locations, Biruni developed a new method of using trigonometric calculations based on the angle between a plain and mountain top which yielded more accurate measurements of the Earth's circumference and made it possible for it to be measured by a single person from a single location.[33][34][35] Biruni's method was intended to avoid "walking across hot, dusty deserts" and the idea came to him when he was on top of a tall mountain in India.[36] From the top of the mountain, he sighted the dip angle which, along with the mountain's height (which he calculated beforehand), he applied to the law of sines formula.[37][36] He also made use of algebra to formulate trigonometric equations and used the astrolabe to measure angles.[38]

"Important contributions to geodesy and geography were also made by Biruni. He introduced techniques to measure the earth and distances on it using triangulation. He found the radius of the earth to be 6339.6 km, a value not obtained in the West until the 16th century. His Masudic canon contains a table giving the coordinates of six hundred places, almost all of which he had direct knowledge."[39]

Geodesy

Geodesy, also called geodetics, is the scientific discipline that deals with the measurement and representation of the Earth, its gravitational field and geodynamic phenomena (polar motion, Earth tides, and crustal motion) in three-dimensional time-varying space.

Geodesy is primarily concerned with positioning and the gravity field and geometrical aspects of their temporal variations, although it can also include the study of Earth's magnetic field. Especially in the German speaking world, geodesy is divided into geomensuration ("Erdmessung" or "höhere Geodäsie"), which is concerned with measuring the Earth on a global scale, and surveying ("Ingenieurgeodäsie"), which is concerned with measuring parts of the surface.

as the shape of Earth's land surface as it rises above and falls below the sea.

As the science of geodesy measured Earth more accurately, the shape of the geoid was first found not to be a perfect sphere but to approximate an oblate spheroid, a specific type of ellipsoid. More recent measurements have measured the geoid to unprecedented accuracy, revealing mass concentrations beneath Earth's surface.

Spherical models

There are several reasonable ways to approximate Earth's shape as a sphere. Most preserve a different feature of an ellipsoid that closely models the real Earth in order to compute the radius of the spherical model. All examples in this section assume the WGS 84 datum, with an equatorial radius a of 6,378.137 km and a polar radius b of 6,356.752 km. A sphere being a gross approximation of the spheroid, which itself is an approximation of the geoid, units are given here in kilometers rather than the millimeter resolution appropriate for geodesy.

Preserve the equatorial circumference. This is simplest, being a sphere with circumference identical to the equatorial circumference of the ellipsoidal model. Since the circumference is the same, so is the radius, at 6,378.137 km.

Preserve the lengths of meridians. This requires an elliptic integral to find, given the polar and equatorial radii: . A sphere preserving the lengths of meridians has a rectifying radius of 6,367.449 km. This can be approximated using the elliptical quadratic mean: , about 6,367.454 km; or even just the mean of the two axes: , about 6,367.445 km.

^ ab Adoption by China via European science: Jean-Claude Martzloff, “Space and Time in Chinese Texts of Astronomy and of Mathematical Astronomy in the Seventeenth and Eighteenth Centuries”, Chinese Science 11 (1993-94): 66-92 (69) and Christopher Cullen, "A Chinese Eratosthenes of the Flat Earth: A Study of a Fragment of Cosmology in Huai Nan tzu 淮 南 子", Bulletin of the School of Oriental and African Studies, Vol. 39, No. 1 (1976), pp. 106-127 (107)

^ Oldenberg (p. 379) places it near the end of the Brahmana period, seeing that the older Brahmanas still contain pre-normalized Rigvedic citations. The Brahmana period is later than the composition of the samhitas of the other Vedas, stretching for about the 10th to 6th centuries. This would mean that the redaction of the texts as preserved was completed in roughly the 6th century BC. The EIEC (p. 306) gives a 7th century date.

^ David A. King, Astronomy in the Service of Islam, (Aldershot (U.K.): Variorum), 1993.

^Gharā'ib al-funūn wa-mulah al-`uyūn (The Book of Curiosities of the Sciences and Marvels for the Eyes), 2.1 "On the mensuration of the Earth and its division into seven climes, as related by Ptolemy and others," (ff. 22b-23a)[2]